This invention relates to superconductive magnet devices provided with magnetic shims for compensating for inhomogeneous magnetic field components and utilized, for example, in a magnetic resonance imaging system.
FIG. 4 is a perspective view of a conventional superconductive magnet device, and FIG. 5 is a side view of the main portion, as extracted from the housing, of the superconductive magnet device of FIG. 4. Within a superconductive magnet body 1 is formed a homogeneous magnetic field region 2. A number of non-magnetic pipes 3 are disposed on inner side of the superconductive magnet body 1 at predetermined circumferential spacings. Rod-shaped magnetic shims 4 are accommodated within the respective non-magnetic pipes 3. Stoppers 5 are inserted into both ends of the non-magnetic pipes 3 to confine the magnetic shims 4 within the non-magnetic pipes 3. The stoppers 5 also serve to position the magnetic shims 4 at predetermined axial positions along the lengths of the non-magnetic pipes 3. The non-magnetic pipes 3 are attached to the superconductive magnet body 1 via attachment frames 6.
It is extremely difficult to attain a sufficient homogeneity within the homogeneous magnetic field region 2 solely by means of the superconductive magnet body 1, due to insufficient manufacturing precision of the superconductive magnet body 1 or adverse effects of magnetic fields generated by neighboring magnetic parts. Thus, the homogeneity of the magnetic field within the homogeneous magnetic field region 2 is improved as follows.
First, the spatial magnetic field distribution within the homogeneous magnetic field region 2 is measured, and, upon the basis of the measurement, the magnitudes of the inhomogeneous magnetic field components are determined. Next, the kinds and number of the rod-shaped magnetic shims 4 required for compensating for the respective inhomogeneous magnetic field components are selected. The selected magnetic shims 4 are inserted into the non-magnetic pipes 3 at the predetermined positions, and are positioned along the lengths of the non-magnetic pipes 3 by means of the stoppers 5.
The spatial magnetic field distribution within the homogeneous magnetic field region 2 is measured again to ascertain whether or not the resulting magnetic field satisfies the prescribed homogeneity. If the homogeneity is below the prescribed level, the above-described steps of measurements and selections of the non-magnetic pipes 3 are repeated until the prescribed homogeneity level is attained.
By the way, in the above-described steps of inhomogeneous magnetic field compensation, the kinds and the number of the magnetic shims 4 inserted into the non-magnetic pipes 3 and the positions of the magnetic shims 4 within the non-magnetic pipes 3, etc., are recorded after every step, such that the compensation procedures up to the present can be comprehended at once.
The above conventional superconductive magnet device thus has the following disadvantage. Namely, during the compensation procedure of the inhomogeneous magnetic fields, the kinds and number of the magnetic shims 4 must be recorded precisely every time the magnetic shims 4 are changed and inserted into the non-magnetic pipes 3. Further, if the record contains an error, all the magnetic shims 4 must once be removed from the non-magnetic pipes 3 to confirm the kinds and the number of the magnetic shims 4. When the magnetic shims 4 are thus removed, it is necessary to ascertain the position of the magnetic shims 4 within the non-magnetic pipes 3. To do this, the operator must look into the non-magnetic pipes 3 from their ends. This is dangerous since the magnetic shims 4 may jump out of the non-magnetic pipes 3 due to the attraction of the magnetic field generated by the magnet.